The overall objective of this application is to elucidate underlying molecular mechanisms involved in cardiac function and heart formation. Congenital cardiovascular anomalies are the most common form of human birth defect with a recorded instance of 1 per 200 liver births per year in North America. There is therefore considerable interest in understanding the molecular and genetic bases of these diseases. Recent evidence in rodent systems indicates that the serum response factor (SRF), a member of the MADS (MCMI, Agamous and Deficiens, SRF) box family of transcription factors, is a critical regulator of cardiac development and function. SRF has been shown to regulate various cardiac and skeletal muscle specific genes necessary for normal cardiac development and function, including the cardiac and skeletal actin, dystrophin, myosin light chain and atrial natriuretic peptide genes. Consistent with this, cardiac specific over-expression of SRF in transgenic animals results in reinduction of an embryonic program of gene expression that can lead to dramatic cardiac hypertrophic and myopthic phenotypes that mimic those observed during the initial development of congestive heart failure in humans. However, knock-out of the SRF gene is embryonic lethal prior to cardiac differentiation. Therefore, despite a central place for SRF in heart function and development the role of SRF in heart formation in vivo has not been carefully investigated. In the current application we propose to study the role of SRF during early cardiogenesis using a powerful novel transgenic approach in which targeted gene insertion of dominant inhibitory versions of SRF is coupled with embryonic stem cell aggregation techniques. The results changes in SRF target gene expression and cardiac morphology of early stage embryos will then be analyzed. These studies will both elucidate the mechanisms by which SRF functions in heart and will establish powerful new methodologies for studying heart formation and function.